The rapid development of nanoparticles (NPs) has raised significant concerns regarding their potential toxic effects on both human health and the environment. Zebrafish (Danio rerio) have emerged as a valuable in vivo model for nanotoxicity studies due to their physiological and genetic similarities to humans, high fecundity, transparent embryos, and rapid development. This chapter provides a comprehensive review of zebrafish as a model organism for investigating NP toxicity. It highlights key advantages, including their suitability for high-throughput screening and real-time visualization of NP biodistribution. The chapter discusses various NP uptake pathways, such as the gills, gastrointestinal tract, and blood-brain barrier, and explores the biological barriers that influence NP accumulation. Furthermore, it summarizes toxicological findings on teratogenic, immunotoxic, neurotoxic, and hepatotoxic effects of NPs across different zebrafish life stages. The use of zebrafish allows for the investigation of both acute and chronic NP exposure, offering insights into developmental and reproductive toxicity, oxidative stress, and genotoxicity. By bridging the gap between simple in vitro tests and more complex mammalian models, zebrafish serve as an essential model for assessing the potential risks of nanomaterials for human and environmental health.